The present invention relates to a cellulosic substrate suitable for use as an envelope or mailer and, more particularly, to one having at least one transparentized portion.
As is known, various types of envelopes or mailers with transparent windows exist where the window consists of a cut-out opening in the mailer substrate which is covered by a transparent patch. The transparent patch is usually secured over the cut-out opening by means of an adhesive, and may consist of any suitable film of transparent material such as glassine, cellophane, or polymeric materials including polyester, polyethylene, polycarbonate, polystyrene, and polyethylene terephthalate. The adhesive is generally applied to the mailer substrate around the perimeter of the cut-out opening to join the outer perimeter of the transparent patch thereto. The transparent patch can be secured to either the inside or outside surface of the mailer substrate.
In some modern mailing systems, a mailer is formed from a single sheet after it has been imaged by a non-impact printer. These sheets are stacked in the input hopper and fed as single plies through the printer, after which the sheets are folded to form a mailer. A window is provided to permit the name and address to show through. Added thickness caused by patches over diecut windows causes mis-shapen stacks and prevents trouble-free feeding.
The typical arrangement of such patches is disadvantageous in that, since the transparent patch is layered on top of or below the substrate, the thickness of the window portion of the sheet is greater than that of the remainder of the sheet. As a consequence, such sheets form unstable and uneven stacks, and thus limit the maximum height to which they can be stacked. This stack-height limitation is burdensome to large scale manufacturing operations, such as printing.
Another disadvantage with mailers having a cut-out opening covered by a transparent patch is that the edges of the transparent patch often get caught by process machinery, such as printers. This results in the destruction of the mailer and usually requires the machinery to be stopped so that the destroyed mailer can be removed. Moreover, when heat is employed in such process machinery, the adhesive holding the transparent patch to the mailer substrate can soften, causing the patch to become detached from the mailer substrate.
One alternative to the cut-out/transparent patch type of arrangement is to apply a transparentizing material to a predetermined portion of the mailer substrate to thereby form a window. Such a method entails the impregnation of the mailer substrate with transparentizing material. The spaces between the fibers of the substrate are filled by the transparentizing material. In order to make the impregnated portion transparent, the transparentizing material must have a refractive index close to that of cellulose (1.5). Examples of conventional transparentizing methods and materials are disclosed in U.S. Pat. No. 3,813,261 to Muller, U.S. Pat. No. 4,137,046 to Koike et al., and U.S. Pat. No. 4,198,465 to Moore et al.
In order to produce high quality mailers on a large, industrial scale by employing a transparentizing material, it is desirable that the transparentizing material be capable of achieving three important functions:
1) the ability to quickly penetrate the mailer substrate in order to fully impregnate the substrate in the shortest time possible; PA0 2) the ability to be converted quickly from a penetrating liquid to a solid after impregnation has occurred; and PA0 3) the ability to produce a transparentized portion which possesses a number of physical and chemical properties, the details of which will be discussed below. The drawback to producing mailers in this manner, however, is that currently available transparentizing materials can perform, at most, only one of the aforementioned functions.
The rate at which some conventional transparentizing materials penetrate a cellulosic substrate is so slow that, after applying the transparentizing material to the substrate, the substrate must be wound up in a tight roll for a period of time to allow the material to impregnate the substrate. See, for example, U.S. Pat. No. 4,416,950 to Muller et al. Such materials are not conducive to the high-speed production of mailers having transparentized windows.
It is known to include a solvent with the transparentizing material to lower the viscosity thereof and thereby speed the rate of penetration of the transparentizing material into the mailer substrate (see, e.g., U.S. Pat. No. 4,513,056 to Vernios et al). However, the use of solvents with transparentizing materials is undesirable due the added process machinery and expense which are necessary to evaporate the solvent from the substrate surface and to recover the evaporated solvent.
As mentioned, the transparentized portion of a mailer preferably should possess certain physical and chemical properties. Physically, the transparentized portion should be strong and flexible (i.e. not brittle) and be receptive to inks. Chemically, the transparentized portion should have sufficient resistance to ultraviolet radiation that it does not yellow and/or lose its transparency over time; should meet U.S. Postal Service specifications for reflectance (sufficient transparency to read the printing beneath the transparentized portion) and PCR ("Print Contrast Ratio"--sufficient contrast between the printing and background beneath the transparentized portion); and should have sufficient resistance to migration and/or volatilization of the transparentizing material from the place where applied on the mailer substrate such that it does not lose its transparency over time.
Conventional transparentizing materials which may be capable of somewhat faster penetration rates are not capable of producing transparentized portions which possess the aforementioned physical and chemical properties. U.S. Pat. No. 5,076,489 to Steidinger, for example, discloses using either wax or oil as the transparentizing material. Wax produces a brittle transparentized area which is easily marred by physical contact therewith to cause a loss of transparency. In addition, wax is not receptive to inks and therefore cannot be printed upon. Oil tends to migrate and/or volatilize easily, thus resulting in a loss of transparency over time.
As a further example, Lombardi et al, U.S. Pat. No. 4,237,185, discloses a transparentizing material containing an aromatic acrylate oligomer (an acrylated monofunctional epoxidized novolac). It has been found that when oligomers (or monomers) having an aromatic structure are included in a transparentizing material, the transparentized area produced thereby yellows and/or loses its transparency over time.
Accordingly, it is seen that a need exists in the art for a substrate suitable for use as a mailer or envelope having at least one transparentized portion which can be placed in tall, stable stacks; which does not have equipment-catching edges around the window area; and which, if produced by transparentizing a portion of the mailer substrate, can be transparentized quickly and yet produce a transparentized portion which has the desired physical and chemical properties recited above.